Modulation methods adopted by wireless communications such as cellular phones of recent years have high frequency utilization efficiency and at the same time, high peak to average power ratio (PAPR: Peak to Average Power Ratio). In order to amplify a signal to which an amplitude modulation is applied using a class AB amplifier used in the field of wireless communications before now, it is necessary to secure enough back-off to maintain linearity. Generally, this back-off needs to be as much as PAPR at least. In contrast, efficiency of class AB amplifiers becomes highest at the time of power saturation, and declines as back-off becomes larger. For this reason, the larger the PAPR of a high frequency modulation signal becomes, the more difficult to raise power efficiency of a power amplifier will be.
As a power amplifier which amplifies such a modulation signal of large PAPR with high efficiency, there is a polar modulation-type power amplifier. In the polar modulation-type power amplifier, the high frequency modulation signal used for wireless communication is generated from component of polar coordinates of amplitude and phase.
FIG. 6 is a block diagram of a polar modulation-type power amplifier disclosed in non-patent document 1. The amplifier concerned includes: a high frequency modulation signal input terminal 101, an amplitude signal input terminal 102, a power supply circuit 103, a high frequency power amplifier 104 and a high frequency modulation signal output terminal 105. Also, the power supply circuit 103 includes: a linear amplifier 106, a subtractor 107, a current sensing resistor 108, a hysteresis comparator 109, a switching amplifier 110, an inductor 111 and a power supply terminal 112.
From the high frequency modulation signal input terminal 101, a high frequency modulation signal to which an amplitude modulation or a phase modulation is applied is input, and sent to the high frequency power amplifier 104. From the amplitude signal input terminal 102, an amplitude signal of the high frequency modulation signal, which is input from the high frequency modulation signal input terminal 101, is input. The signal input from the amplitude signal input terminal 102 is amplified with high efficiency in the power supply circuit 103, and is supplied from the power supply terminal 112 as a power supply of the high frequency power amplifier 104. The high frequency power amplifier 104 amplifies the signal input from the high frequency modulation signal input terminal 101 and outputs it to the high frequency modulation signal output terminal 105.
The power supply circuit 103 has a structure including the linear amplifier 106 in combination with the switching amplifier 110 in order to amplify the input signal with high efficiency and low distortion. The amplitude signal input from the amplitude signal input terminal 102 is input to the linear amplifier 106.
The linear amplifier 106 has low output impedance, and performs linear amplification of the input signal and outputs the input signal. The signal output from the linear amplifier 106 is sent to the power supply terminal 112 via the current sensing resistor 108.
The subtractor 107 is connected to both ends of the current sensing resistor 108, and outputs a value which is subtracted a voltage of the power supply terminal 112 from a voltage of the output signal of the linear amplifier 106. At that time, since the input of the subtractor 107 is at high impedance, there will be no case where the subtractor 107 consumes the electric power supplied to the output signal of the linear amplifier 106 and the power supply terminal 112 largely.
Also, since impedance of the current sensing resistor 108 is set low, a voltage which is applied to both ends of the current sensing resistor 108 is negligibly small compared with the voltage applied to the power supply terminal 112.
The output signal of the subtractor 107 is input to the hysteresis comparator 109. The hysteresis comparator 109 performs plus or minus judgment of the input signal and outputs the result (pulse signal) to the switching amplifier 110. However, the hysteresis comparator 109 has a function to hold the last output state and hysteresis width (V_hys), and when the last output is Low, the output reverses to High when the input signal becomes not less than V_hys/2, and when the last output is High, the output reverses to Low when the input signal becomes not more than −V_hys/2.
The signal input to the switching amplifier 110 is amplified, and is output to the power supply terminal 112 via the inductor 111. At that time, an electric current supplied from the switching amplifier 110 via the inductor 111 is combined with an electric current supplied from the linear amplifier 106 via the current sensing resistor 108 and sent to the power supply terminal 112.
The power supply circuit 103 mentioned above has two advantages; linearity of the linear amplifier 106 and high efficiency of the switching amplifier 110. This is because, in the power supply circuit 103, the linear amplifier 106 of low output impedance decides the output voltage and most part of the output current is supplied from the switching amplifier 110 with high efficiency. The electric current output from the power supply terminal 112 is a sum of the output current of the linear amplifier 106 and the output current of the switching amplifier 110. An electric potential of the power supply terminal 112 is decided by the linear amplifier 106 with low output impedance. In order to keep the electric potential of the power supply terminal 112 at a value of a target, an electric current is supplied from the linear amplifier 106. The output current of the linear amplifier 106 is detected by the current sensing resistor 108 and the hysteresis comparator 109, and a supply current from the switching amplifier 110 is adjusted so that the output current of the linear amplifier 106 does not become excessive. By applying the method mentioned above, most of the electric current output from the power supply terminal 112 is supplied from the switching amplifier 110, and it is enough for the output current of the linear amplifier 106 only to correct an error component of the switching amplifier 110.
FIG. 7 is a block diagram of a power supply circuit disclosed in non-patent document 2. The power supply circuit includes: a signal input terminal 701, a linear amplifier 702, a current detector 703, amplifiers 704, 705 and 707, an adder 706, a PWM modulator 708, a switching amplifier 709, an inductor 710 and a signal output terminal 711. In the above, PWM is an abbreviation of Pulse Width Modulation.
A signal input from the signal input terminal 701 is supplied to the linear amplifier 702 and the amplifier 704. The linear amplifier 702 amplifies the signal supplied to the signal input terminal 701 and outputs the signal to the signal output terminal 711. The current detector 703 detects a current value of the signal output from the linear amplifier 702. The amplifier 704 adjusts amplitude of the signal supplied to the signal input terminal 701 and outputs the signal supplied to the signal input terminal 701. The amplifier 705 adjusts amplitude of the signal detected by the current detector 703 and outputs the signal detected by the current detector 703. The adder 706 calculates a sum of the output signal of the amplifier 704 and the output signal of the amplifier 705, and outputs the sum. The amplifier 707 adjusts amplitude of the signal output by the adder 706 and outputs the signal output by the adder 706. The PWM modulator 708 converts the output signal of the amplifier 707 into a 1 bit signal of PWM and outputs the 1 bit signal. The switching amplifier 709 amplifies the output signal of the PWM modulator 708 and outputs the output signal of the PWM modulator 708 to the signal output terminal 711 via the inductor 710. At that time, the current of the output signal of the switching amplifier 709 and the output signal of the linear amplifier 702 is combined.
Improvement is applied to non-patent document 2 on the basis of the circuit of non-patent document 1 so that the non-patent document 2 can perform control of the switching amplifier by the PWM modulation.